Light emitting diode (LED) technology is a maturing technology that continues to show improvements in efficiency, customability and cost reduction. LED technology is rapidly being deployed in a host of industries and markets including general lighting for homes, offices, and transportation, solid state display lighting such as in LCDs, aviation, agricultural, medical, and other fields of application. The increased energy efficiency of LED technology compared with other lighting solutions coupled with the reduction of costs of LED themselves are increasing the number of LED applications and rates of adoption across industries. While LED technology promises greater reliability, longer lifetimes and greater efficiencies than other lighting technologies, the ability to mix and independently drive different color LEDs to produce customized and dynamic light output makes LED technology and solid-state lighting (SSL) in general robust platforms to meet the demands of a variety of market needs and opens the door to many new applications of these lighting technologies.
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and is found in intrinsically photosensitive retinal ganglion cells (ipRGCs) of humans and other mammals. Melanopsin plays an important non-image-forming role in the photoentrainment of circadian rhythms as well as potentially many other physiologic functions. Stimulation of melanopsin-containing ipRGCs contributes to various reflexive responses of the brain and body to the presence of light. Melanopsin photoreceptors are sensitive to a range of wavelengths and reach peak light absorption at wavelengths around 480-500 (or 490) nanometers (nm). Melanopic light, that is light corresponding to the melanopsin action spectrum, including particularly the wavelengths in the 480-500 nm region, is important for non-visual stimuli including physiological and neurological effects such as pupillary light reflex and circadian entrainment and/or disruption. Time-coordinated exposure, including over-exposure and under-exposure to melanopic light, can be used to entrain and facilitate healthy circadian rhythms in humans and other mammals.
Circadian related photoreceptors are in the macular and peripheral vision regions of the retina. Melanopsin related photoreceptors are most sensitive in the lower hemisphere of the retina. Selective stimulation of these photoreceptors is possible by directing illumination, and specifically melanopic light, towards or away from the region of the retina where melanopic photoreceptors are most concentrated or most sensitive or responsive. If the desire is to optimally stimulate these photoreceptors, then a light source that directs biological light (i.e., melanopic light) onto this region would be a good solution. Equivalent Melanopic Lux (EML) is a metric for measuring the biological effects of light on humans. EML as a metric is weighted to the ipRGCs response to light and translates how much the spectrum of a light source stimulates ipRGCs and affects the circadian system. Melanopic ratio is the ratio of melanopic lux to photopic lux for a given light source.
While it is well known that exposure to light, both natural and artificial, can affect an individual's circadian rhythms, studies also indicate that the natural light of the sky during twilight, that is the short period around dawn or dusk when the sun is near the horizon, may have a significant impact on circadian drive and/or the gating of sleep pressure. Although the sky appears deep blue during twilight, it has significantly less radiant energy in the melanopic region (e.g., 490 nm) and significantly higher radiant energy in the 420 nm region, as compared to the sky during midday.
Scientific data indicates that the suprachiasmatic nucleus contains color representation of the sensed color of light. During the vast majority of the daytime, when the sun is up, the color temperature of the sky is between 5500 K and 7000 K. The only time when this changes is during twilight periods when the sun is low. Common perception suggests that at these times the sky gets redder. However, this is not the case, and while the sun appears redder as its irradiance travels through more of our earth's atmosphere, in fact the sky gets much bluer (e.g., at twilight, the color temperature of the sky may be at 8000-9000 K).
There are two unique and compelling circadian phenomena which coincide with the time when the sky gets bluer. First, sleep inertia, which is the tendency for humans to remain asleep, occurs during sleep. Upon wakening, a circadian-driven surge in blood cortisol levels helps individuals to wake up refreshed by mitigating sleep inertia. This cortisol response has been shown to synergistically occur with presence of light. On the other end of the day, e.g., at sunset, the wake maintenance zone portion of the circadian cycle has been demonstrated as a point of hyperactivity and enhanced neurobiological activity. It is hypothesized that this heightened activity may be an evolutionary survival response to ensure that individuals have sufficient alertness and energy to complete any tasks and find safety prior to the onset of darkness. At the time of day around twilight (or equivalent point in a circadian photoperiod) the human neurophysiology may be affected by specific light cues (that occur only at twilight) with regard to the body's circadian rhythm. For example, one effect may be the initiation of a sleep gating process or conversely the absence or reduction of such gating without exposure to the twilight.
The ability to control the circadian spectra and color temperatures of light during dimming of LED fixtures, such as in response to an environmental cue or to human preference, has been implemented in some systems. For example, systems have been disclosed in which a user can control the amount of circadian stimulation depending on the time of day or based on certain activities such as working late or jet-lag. Existing systems often include individual controllers to adjust the color temperature of the light and overall brightness such as through dimmers. Systems also have included learning modes to understand a user's habits during the day or to learn behavior according to certain activity cues.